1. FIELD OF THE INVENTION
The present invention relates to a method of producing an ultra-low-carbon steel through a vacuum decarburization process. More particularly, the present invention is concerned with a method of producing an ultra-low-carbon steel in which non-deoxidized or weakly-deoxidized molten steel prepared by a steel making furnace, particularly a combined blowing converter or an LD converter, is decarburized by a vacuum degasser, whereby an ultra-low-carbon steel having a carbon concentration less than 10 ppm can be produced quickly without impeding operation of a vacuum degassing plant.
2. DESCRIPTION OF THE RELATED ART
A continuous annealing apparatus, which has become available in recent years, has created a remarkable increase in the productivity of cold-rolled steel strip. This continuous annealing system has given a rise to the demand for an ultra-low-carbon steel having a carbon content of 10 ppm or less.
Conventionally, an ultra-low-carbon steel has been produced by a process in which a molten steel, which has been decarburized in a converter down to 0.02 to 0.05 wt% in terms of carbon content, is exposed to a low pressure atmosphere in a vacuum degasser such as a RH degasser so that carbon is extracted as CO gas. With this known method relying upon a vacuum degasser, however, it has been difficult to produce an ultra-low-carbon steel having a carbon content [C] less than 10 ppm in an industrial scale, because the decarburization rate is drastically decreased when the carbon content [C] is reduced to a level less than 50 ppm.
In order to accelerate the decarburization rate in such low carbon region, it has been considered significant to increase the area of the reaction site. With this knowledge, it has been attempted to enhance the reaction rate by increasing the area of the reaction site. Gas bubbles in molten steel, or surface of the molten steel in a vacuum chamber, or splash metal in the vacuum chamber is considered reaction site. Thus far, the extent of contribution of each of such reaction sites to the reaction has not been definitely determined. Under these circumstances, a method employing blowing of Ar gas into molten steel in an RH vacuum chamber at a large rate of 5 Nm.sup.3 /min or so has been used with a view that an increase in the flow rate of Ar as agitating or recirculating gas would contribute to promotion of decarburization reaction.
Blowing of Ar gas at such a large rate, however, causes a problem in that the degasser cannot operate continuously due to deposition of splash metal to the inner surface of the vacuum chamber of the vacuum degasser as a result of vigorous generation of splash metal caused by the blowing of Ar gas.
In order to obviate the above-described problem, a method has been proposed and used in which hydrogen gas or a hydrogen-containing gas is blown into a molten steel so as to increase the content of hydrogen dissolved in the molten steel [H]. According to this method, a reaction expressed by 2H.fwdarw.H.sub.2 takes place to generate bubbles of hydrogen gas so as to enhance the effect of agitation and to increase the decarburization rate by the increase in the area of the reaction sites. This method is disclosed in Japanese Patent Laid-Open No. 57-194206.
It has been confirmed that this method can increase the decarburization rate in the low-carbon region and, hence, contributes to improvement in the efficiency of production of ultra-low-carbon steel. This method, however, requires that the hydrogen content is maintained at a sufficiently high level, e.g., 3 to 5 ppm, in order to provide an appreciable effect in promoting decarburization. To maintain such a high hydrogen content, it has been required that hydrogen is blown at a rate not smaller than 2.5 Nm.sup.3 /min, when an RH degasser having a capacity of, for example, 250 tons is used.
For the sake of effective production of ultra-low-carbon steel, the pressure in the vacuum chamber is generally reduced to less than 2 Torr. On the other hand, the reduction in the pressure in the vacuum chamber leads to a significant promotion of dehydrogenation reaction, making it difficult to maintain the hydrogen content at a considerably high level.
Thus, the known process of employing an RH conventional vacuum degasser requires an impracticably long time, e.g., 30 to 40 minutes or longer, of decarburization for reducing the carbon content to a level below 10 ppm, even when the recirculation velocity is increased for the purpose of accelerating the decarburization reaction.